Implants which are used for insertion into bones, such as, for example, hip or knee joint prostheses or pins to be screwed into the jaw to construct artificial teeth, are known per se. Such implants preferably consist of titanium or titanium-based alloys such as, for example, titanium/zirconium alloys, it being possible for the latter additionally to contain niobium, tantalum or other tissue-compatible metallic additions. The central properties of such implants are the strengths of the anchoring in the bone and the period of time in which integration is achieved. Osteointegration accordingly means a frictionally solid and permanent connection between implant surface and bone tissue.
The firmness of the anchoring of the implant in the bone can be established by mechanical measurements, namely by measuring the force, whether as pulling, pushing, shearing or torque, which is necessary in order to extract or unscrew the implant anchored in the bone from its anchoring, i.e. bring about a break of the adhesion between the surface of the implant and the bone substance connected thereto. Such measurement methods are known per se and described, for example, in Brunski, Clinical Materials, Vol. 10, 1992, pp. 153-201. Measurements have shown only little anchoring of titanium implants with a smooth surface structure in the bone, whereas implants with the roughened surface afford a noticeably improved bone-implant connection in relation to the tenacity.
EP 0 388 576 therefore proposes to apply to the implant surface in a first step a macro-roughness by means of sandblasting, and subsequently to superimpose a micro-roughness on the latter by means of treatment in an acid bath. The implants of this can thus be roughened by means of sandblasting and subsequently be treated with an etching agent, e.g. hydrofluoric acid or hydrochloric acid/sulfuric acid mixture. The surface provided with a defined roughness in this way is then cleaned with solvents and water and subjected to a sterilizing treatment.
The chemical state of the surface of titanium and titanium-based alloys is complex. It is assumed that the surface of titanium metal spontaneously oxidizes in air and water and thus a reaction with water then takes place on the surface, that is to say in the outermost atomic layer of the oxide, with formation of hydroxyl groups. This surface containing hydroxyl groups is referred to in the literature as “hydroxylated” surface. See H. P. Boehm, Acidic and Basic Properties of Hydroxylated Metal Oxide Surfaces, Discussions Faraday Society, Vol. 52, 1971, pp. 264-275.